Known optical transmission systems include a first transmitter transmitting a first optical digital signal and a second transmitter transmitting a second optical digital signal to a first and a second receiver respectively via a common optical transmission link, the second signal being intensity modulated and the second receiver being able to detect intensity modulated signals only.
Such a system, known in the art, is e.g. described in "Siemens Telecom report, 6 Jahrgang, April 1983, Beiheft, p. 122-123". Therein a unidirectional transmission system is shown in FIG. 2b, p. 122, whereas a bidirectional one is represented in FIG. 3b, p. 123. In both systems the transmitters transmit optical intensity modulated signals with distinct wavelengths. These signals are received by so-called direct detection receivers which are sensitive to light intensity variations only. Based on the wavelength of the optical signal each receiver selects the optical signal destined to it by means of a wavelength division demultiplexer.
By using different wavelengths the transmitters can simultaneously send optical signals over the same optical fiber link, but this either requires a relatively wide bandwidth when the used wavelengths are located relatively far from each other, i.e. when plain wavelength multiplexing is used, or expensive transceivers when dense Wavelength Division Multiplexing (WDH) is applied i.e. when the wavelengths are close to each other, which implies the need of a laser with narrow linewidth and very accurate tracking of the emitted optical wavelength.
It is to be noted that in the case of plain wavelength multiplexing the used wavelengths do not have to be very accurate, i.e. a relatively wide spectrum can be used. In what follows such a spectrum is called a wavelength window.